1.1 Field of the Invention
The present invention relates generally to the fields of virology, immunology, disease treatment and prevention. More particularly, it concerns HIV particles with inactivated reverse transcriptase, methods of inactivation, and the use of such particles to prepare components of HIV and to elicit effective immunological responses to HIV. These immune responses are useful in producing diagnostic reagents, assays, and kits for the diagnosis of HIV and related retroviral disease, providing protection from an HIV challenge, and assisting an HIV-infected individual in controlling the replication of the virus. Methods of inactivation are useful for preventing disease through decreasing the risk of infection associated with exposure to HIV infected tissues and materials.
1.2 Description of Related Art
1.2.1 Human Immunodeficiency Virus
Human Immunodeficiency Virus-1 (HIV-1) infection has been reported throughout the world in both developed and developing countries. HIV-2 infection is found predominately in West Africa, Portugal, and Brazil. It is estimated that as of 1990 there were between 800,000 and 1.3 million individuals in the United States that were infected with HIV. An important obstacle to developing a vaccine against HIV is that the mechanism of immunity to HIV infection is ill-understood. Not all of those infected individuals will develop acquired immunodeficiency syndrome (AIDS). Indeed recent reports have suggested that there may be certain individuals that are resistant to HIV-1 infection.
The HIV viruses are members of the Retroviridae family and, more particularly, are classified within the Lentivirinae subfamily. Like nearly all other viruses, the replication cycles of members of the Retroviridae family, commonly known as the retroviruses, include attachment to specific cell receptors, entry into cells, synthesis of proteins and nucleic acids, assembly of progeny virus particles (virions), and release of progeny viruses from the cells. A unique aspect of retrovirus replication is the conversion of the single-stranded RNA genome into a double-stranded DNA molecule that must integrate into the genome of the host cell prior to the synthesis of viral proteins and nucleic acids.
Retrovirus virions are enveloped and contain two copies of the genome. The conversion of the genomic RNA into DNA is provided by the viral protein reverse transcriptase (RT). This protein is bound to the RNA genome within the virion, and its enzymatic conversion of the genome to DNA is believed to take place after viral entry into the host cell. However, recent evidence suggests that the conversion process may initiate in the virion particles themselves, known as endogenous reverse transcription (ER), and that ERT may be important in increasing the infectivity of the virus in sexual transmission (Zhang et al., 1993, 1996).
Because of the requirement for reverse transcription in the viral replication cycle, compounds that interfere with RT activity have been utilized as anti-HIV therapeutic agents. Many of these compounds, including 3′-azido-2′, 3′-dideoxythymidine (AZT), are nucleoside analogs that, upon activation by host cell kinases, are competitive inhibitors of reverse transcriptase (Furman et al., 1986). Other anti-RT compounds are nonnucleoside inhibitors (NNI), hydrophobic compounds that do not require cellular modification for antiviral activity. Examples of such compounds include nevirapine (Grob et al., 1992; Merluzzi et al., 1990), the pyridinones (Carroll et al., 1993; Goldman et al., 1991), and the carboxanilides (Bader et al., 1991; Balzarini et al., 1995, 1996). The nevirapine analog 9-azido-5,6-dihydro-11-ethyl-6-methyl-11H-pyrido[2,3-b][1,5]benzodiazepin-5-one (9-AN) and the carboxanilide analog N-[4-chloro-3-(3-methyl-2-butenyloxy)phenyl]-2-methyl-3-furanocarbothiamide (UC781™) have been shown to be potent inhibitors of RT. In respect of the 9-AN, the exposure of a mixture of this compound and RT to UV-irradiation has been particularly effective in inhibiting RT. From the series of carboxanilides compounds, UC781™ has been found to be particularly effective (Barnard et al., 1997). The addition of a photoreactive label to UC781™ should increase further its ability to inactivate HIV RT, when a mixture of UC781™ and RT is exposed to UV-irradiation. The irradiation of a mixture of a photolabeled NNI of RT and RT is a type of photoinactivation.
The binding affinity and inhibitory effect of UC781™ is so high that the compound was able to eliminate HIV infectivity following short exposure of the isolated virus to UC781™ without the need for photoinactivation (Borkow et al., 1997). Further, this compound was shown to inhibit ERT in HIV virions and, when provided to HIV infected cells, caused the production of noninfectious nascent virus (Borkow et al., 1997). Therefore, it appears that UC781™ is a particularly powerful inactivator of HIV. Although UC781 has been proposed for use in retrovirucidal formulations (Borkow et al., 1997), use as a photoinactivator of HIV for the purpose of producing an non-infectious virus particle useful as an immunogen is absent from the prior art.
Non-infectious, virus-like particles have previously been produced via manipulations of the viral genome. For example, U.S. Pat. No. 6,080,408, incorporated herein by reference, discloses non-infectious virus-like particles wherein RT has been made inactive by virtue of deletions and sequence changes in the viral genome. Rovinski also discloses other methods of making non-infectious, virus-like particles, (or pseudovirions), that involve alterations to other genes of the HIV genome, expression of a subset of HIV virion components, and heat-inactivation of sera from HIV infected individuals. U.S. Pat. No. 6,017,543 discloses methods using formalin, psoralen, beta-propriolactone, alone and in combination, as well as exposure to gamma radiation.
1.2.2 Immune Response to HIV
The immune response to HIV is composed of an initial cell mediated immune response followed by the subsequent development of neutralizing antibodies. Within weeks of infection, virus titers in the blood fall coincident with the induction of anti-HIV cellular and humoral immune responses. The fall in viremia correlates well with the appearance of anti-HIV major histocompatibility complex (MHC) class I-restricted CD8+ cytotoxic T cells (Haynes et al., 1996). Recent evidence has shown a strong correlation of anti-HIV CD4+ T cell responses and reduced viral loads (Rosenberg et al., 1997). Therefore, the presentation of HIV antigens in the context of MHC class II molecules to CD4+ T cells may be the key aspect of the control of the HIV infection.
Rosenberg et al. (1997) suggest that in HIV-1 infection, HIV-specific CD4+ cells may be selectively eliminated. This may be due to the activation of these cells during high-level viremia, increasing their susceptibility to infection (Weissman et al., 1996; Stanley et al., 1996), or may be due to activation induced cell death during primary infection (Abbas, 1996). Nonetheless, increasing the virus-specific CD4+ T cell response without infecting, or destroying, the responding cells may be an effective means of controlling viral loads. Therefore, some existing HIV vaccines may be ineffective because they do not provide presentation of HIV peptides in the context of MHC class II by antigen presenting cells.
1.2.3 HIV Vaccines
Historically, viral vaccines have been enormously successful in the prevention of infection by a particular virus. Therefore, when HIV was first isolated, there was a great amount of optimism that an HIV vaccine would be developed quickly. However, this optimism quickly faded because a number of unforeseen problems emerged. A discussion of the problems that an HIV vaccine must overcome is provided within Stott and Schild (1996) and is incorporated herein by reference.
First, HIV is a retrovirus, thus, during its growth cycle, proviral DNA is integrated in the host genome. In this form the virus is effectively protected from the immune response of the host and this feature of the virus suggests that effective vaccination must ideally prevent the initial virus-cell interaction following transmission. Few, if any, of the currently available successful viral vaccines against other infections achieve this level of protection. Secondly, HIV specifically targets and destroys T-helper lymphocytes, which form an essential component of the immune response. Thirdly, the virus is capable of extremely rapid antigenic variation which permits escape of the virus from immune responses. Fourthly, the majority of infections are acquired sexually via the genital or rectal mucosae, and infections of this route are generally considered difficult to prevent by vaccination. Finally, infection may be transmitted by virus-infected cells in which the proviral DNA is integrated and viral antigens are not expressed. Such a cell would not be recognized by immune responses to viral proteins and would therefore pass undetected. Few data are available to indicate how significant this mode of transmission is in the overall epidemiology of HIV-1. Nevertheless, it represents a potential route and one which some authorities believe cannot be blocked by vaccination (Sabin, 1992).
Types of HIV vaccines include inactivated virus vaccines, live attenuated virus vaccines, virus subunit vaccines, synthetic particle vaccines, and naked DNA vaccines and are reviewed in Stott and Schild (1996), Schultz (1996), and Johnston (1997). Several of these vaccines are already in human trials.
The first evidence that vaccination against immunodeficiency viruses was feasible came from early experiments using simple inactivated virus prevented the onset of disease when vaccinated animals were subsequently challenged (Desrosiers et al., 1989; Sutjipto et al., 1990). These results were confirmed and extended by Murphey-Corb et al. (1989) who showed that most animals immunized with formalin-inactivated virus were protected against infection with SIV. Similar results were subsequently obtained by several laboratories using virus-infected cells (Stott et al., 1990) or partially purified virus, inactivated by aldehydes (Putkonen et al., 1991, 1992; Johnson et al., 1992a; Le Grand et al., 1992), β-propiolactone (Stott et al., 1990) detergent (Osterhaus et al., 1992) or psoralin and UV light (Carlson et al., 1990). Several different isolates of SIV or infectious molecular clones derived from them were used to prepare the vaccine and challenge viruses. A wide variety of adjuvants were also employed. On every occasion vaccinated macaques were protected against infection by intravenous challenge of between 10-50 MID50 (50% monkey infectious doses). Infections virus could not be recovered from the blood or tissues of the protected animals even when they were followed for prolonged periods of over 1 year. Even more impressive was the failure to detect proviral DNA in the lymphocytes of protected animals, indicating that there had been no integration of the challenge virus (Stott et al., 1990; Johnson et al., 1992a). It was thus clear that inactivated virus vaccines induced a powerful protective response in macaques. Unfortunately, the protection induced by inactivated SIV in macaques was not reproduced in chimpanzees vaccinated with inactivated HIV and challenged with HIV-1 (Warren and Doltshahi, 1993).
1.2.4 Photoinactivation of HIV
Methods of photoinactivation of HIV are known in the art and have been the subject of at least three patents. U.S. Pat. No. 5,041,078 describes the use of sapphyrins in the photodynamic inactivation of viruses, including HIV. U.S. Pat. Nos. 5,516,629 and 5,593,823 describe the use of psoralens and ultra violet light to inactivate HIV. U.S. Pat. No. 5,516,629 is incorporated herein by reference. Psoralens are naturally occurring compounds which have been used therapeutically for millennia in Asia and Africa A psoralen binds to nucleic acid double helices by intercalation between base pairs. Upon absorption of UVA photons, the psoralen excited state has been shown to react with a thymine or uracil double bond and covalently attach to both strands of a nucleic acid helix. The crosslinking reaction is specific for a thymine (DNA) or uracil (RNA) base and will proceed only if the psoralen is intercalated in a site containing thymine or uracil. The initial photoadduct can absorb a second UVA photon and react with a second thymine or uracil on the opposing strand of the double helix to crosslink the two strands of the double helix.
Lethal damage to a cell or virus occurs when a psoralen intercalated into a nucleic acid duplex in sites containing two thymines (or uracils) on opposing strands sequentially absorb 2 UVA photons. This is an inefficient process because two low probability events are required, the localization of the psoralen into sites with two thymines (or uracils) present and its sequential absorption of 2 UVA photons.
Attempts to inactivate viruses using photosensitizers and light have also been developed using some non-psoralen photosensitizers. The photosensitizers that have been employed are typically dyes. Examples include dihematoporphyrin ether (DHE), Merocyanine 540 (MC540) and methylene blue.
Carlson et al. (1990) has shown that a psoralen-inactivated whole SIV (the Simian counterpart of HIV) vaccine can protect against low challenge doses of SIV and prevent early death in those monkeys that do become infected, suggesting that inactivated HIV may be an effective vaccine in humans. However, because photoinactivation using psoralens is dependent on two rare events, a more efficient method of inactivation is preferable to decrease the likelihood of live virus within a sample. Furthermore, these methods alter the antigenic conformation of HIV affecting the production of an effective immunological response.
1.3 Deficiencies in the Prior Art
Due to previous successes in preventing viral diseases using subunit, live-attenuated viral, and inactivated viral vaccines, the scientific community was initially optimistic that a vaccine would be developed to prevent the spread of HIV. However, early optimism soon diminished because of repeated failures in the development of an effective vaccine.
Vaccines based on subunit immunogens, although extremely safe, are limited in the breadth of antigens that are presented to the immune system because only one or a few of the viral proteins are utilized as immunogens. This may limit the likelihood of cross protection between clades of HIV. Also, the production of vaccines based upon subunit immunogens requires the molecular manipulation of the viral proteins into cloning or expression vectors, perhaps leading to increased production time and costs.
Live-attenuated HIV vaccines typically utilize whole, or nearly whole virions as immunogens. Production of these vaccines may require molecular manipulations of the HIV genome in their production, although spontaneous attenuated viruses may occur naturally. Attenuated HIV vaccines have included deletions in the nef region of the virus. Mutant-nef SIV vaccines showed initial promise in primates, however, it was quickly shown that these vaccines were capable of causing disease in newborn animals. Furthermore, recent evidence suggests that these vaccines are capable of causing full-blown AIDS in adult monkeys (Cohen, 1997). Therefore, the lack of an efficient understanding of HIV and its pathogenesis makes the use of attenuated viruses a risky endeavor.
Inactivated viral vaccines provide a larger compliment of immunogens presented to the immune system, and, therefore, provide a greater amount of protection from HIV and are more likely to provide protection across HIV clades. Furthermore, the inactivated viral vaccines do not require molecular manipulation of HIV and can be made to essentially any strain. Because inactivation of the virus is readily shown in in vitro and animal models, the inactivated HIV vaccines are able to be tested in a timely manner to determine the effectiveness of inactivation. Attenuated viruses may take years to determine the effectiveness of the vaccine.
To be safe to administer to humans, efficient methods of inactivation of HIV are required for vaccine production. Methods known for the inactivation include the use of aldehydes, β-propiolactone, psoralin and UV light, and others including detergents. Many of these methods alter the conformation of the virus thereby altering the specificity of the immune response to the virus. Even gamma radiation may require the additional use of protective compounds to help preserve viral protein integrity, as disclosed in U.S. Pat. No. 6,017,543. Photoinactivation of HIV using psoralin and UV light does not alter the conformation of the virus, but it is an inefficient method of inactivation. Therefore, more efficient methods of inactivation that do not affect the conformation of the viral immunogen would be ideal for use in the production of an HIV vaccine.